Transmission Method and Related Base Station

ABSTRACT

The invention relates to transmission from a base station to a receiver, the base station comprising first radiating means arranged for transmitting according to an antenna pattern with respect to the receiver and further comprising second radiating means arranged for transmitting according to the same antenna pattern as the first radiating means with respect to the receiver, the transmissions of the first and second radiating means being time shifted by a determined duration.

BACKGROUND OF THE INVENTION

The present invention relates to transmission from a base station to areceiver.

It can apply to a base station of a radiocommunication network, such asa GSM (“Global System for Mobile communications”) or a UMTS (“UniversalMobile Telecommunication System”) network for instance. Of course, itcan also apply to other types of base stations.

The receiver can be a mobile terminal, but also any other type ofreceiver, such as another base station, a base station controller, aradio network controller, etc.

It is known that communication by radio between a base station and amobile terminal for instance, is subject to phenomena that disturb theradio transmission between the antenna of the base station and theantenna of the mobile terminal, in particular to channel fadings due todestructive interference between signals which follow differentpropagation paths between the base station and the terminal.

The diversity of one of the characteristics related to this transmissionis one of the methods developed for alleviating fading. Thus, use ismade of transmission diversity, consisting in equipping the base stationwith several antennas transmitting the same signals, polarizationdiversity, frequency diversity (see for example the work “Reseaux GSM”[GSM networks] by X. Lagrange et al, published by Hermes SciencePublications, 2000, page 161), etc.

It is known practice to use antennas comprising devices for altering theradiation pattern. Such adjustments pertain for example to the directionof transmission of the antenna or the width of the main transmissionlobe.

These alterations of the radiation pattern may be mechanical, such asthe orienting of an antenna arranged on an articulated support, mixedelectrical/mechanical (cf. U.S. Pat. No. 6,198,458), or else purelyelectronic, as in FR-A-2 792 116 or its US equivalent U.S. Pat. No.6,480,154.

Most antennas with electronic steering of the beam are composed ofseveral antenna elements individually fed with signals obtained by phaseshifting an initial signal. The value of the phase shift is determinedas a function of the antenna element to which the phase-shifted signalis addressed, and the direction of transmission by the antenna resultsfrom the combining of the mutually phase-shifted signals transmitted byall the antenna elements.

Such antennas, also known as “smart antennas”, are sometimes used tofocus a radio beam intended for a particular terminal. In a particularembodiment, the components of an uplink radio signal transmitted by theterminal and which are picked up by the various antenna elements, areanalyzed in terms of phase shifts so as to estimate a direction in spacefrom which this uplink signal originates. Corresponding phase shifts arethen applied to the downlink signal intended for this terminal so thatits transmission is oriented in this direction. Such electronic steeringof the beam allows considerable reductions in interference level.

A base station with such antenna is described below with reference toFIG. 1 and FIG. 2.

In FIG. 1, a base station 100 transmits by means of the antenna 1, aradio signal intended for a terminal 200 situated within range of thisantenna. In this example, the antenna 1 consists of juxtaposed radiatingelements 2. All these radiating elements 2 are fixed with respect to thesupport of the antenna 101, and oriented facing the geographical sectorintended to be served by the antenna.

The transmission pattern of the antenna generally consists of a mainlobe, corresponding to an angular sector inside which the radiationpower is greater than a fixed value, and limited according to theseparation with respect to the antenna by the reduction in power relatedto the propagation of the radiation. The axis of this main lobecorresponds to the direction D of transmission of the antenna 1.

The direction D of transmission can be charted by a system of sphericalcoordinates having as pole the centre O of the antenna 1. Thesecoordinates comprise for example the angle of elevation of the directionD of transmission with respect to a horizontal plane containing thepoint O, and the angle of azimuth between the projection of thedirection D onto the horizontal plane and a reference axis R containedin this plane, for example oriented perpendicularly to the grouping ofradiating elements and passing through the point O.

Fluctuations in the direction D of transmission of the antenna 1, e.g.due to movement of the mobile terminal 200, are then charted through theevolution of the angles of elevation and of azimuth. Thus, a fluctuationin the direction D lying in a vertical plane corresponds to a variationin the angle of elevation. A fluctuation lying in a horizontal planecorresponds to a variation in the angle of azimuth.

In most digital radiocommunication systems, the signals are transmittedafter application of a channel coding and of an interleaving. Thechannel coding adds redundancy to the symbols of the digital signal,with a structure allowing the receiver to detect and correct thetransmission errors. The codes customarily employed have optimalperformance when the errors arising in the course of transmission areuncorrelated. The interleaving consists of a permutation of the symbolsthat is intended to tend towards this condition of non correlation whilethe transmission errors on a radio interface have a tendency rather moreto arise through packets on account of the fading phenomenon. Thepermutation of the interleaving pertains to a certain duration (of a fewtens of milliseconds) chosen to achieve a compromise between theperformance of the decoder and the processing delay which theinterleaver entails. This interleaving duration may vary from onechannel to another, such as for example in the case of a UMTS(“Universal Mobile Telecommunication System”) system where it is from 10to 80 ms.

FIG. 2 diagrammatically shows an example of the means employed by a basestation to adapt the antenna pattern, in order to focus it in aparticular direction of transmission. Each signal component S₁, S₂, . .. S_(M), intended for a particular terminal 200 or one belonging to acommon channel, is produced by a processing pathway comprising a channelcoder 3, an interleaver 4, a modulator 5, then a power adjustment module6. The signal components S₁, S₂, . . . , S_(M), delivered by the variousprocessing pathways are subsequently combined by a multiplexing unit 7into a baseband signal S delivered to the radio transmission stage.

The makeup of the modulators 5 and of the multiplexing unit 7 depends onthe multiple access mode employed in the radiocommunication system towhich the invention is applied. In a system where the multiple access isby time division (TDMA), as for example GSM, the modulators 5 carry outthe modulation in baseband or on an intermediate frequency, whereas themultiplexer 7 distributes the signal components S₁, S₂, . . . , S_(M),into respective time slots of the signal frames, corresponding to thevarious channels. In a system where the multiple access is by codedivision (CDMA), such as for example UMTS, the modulators 5 can carryout the spectrum spreading by applying the spreading codes assigned tothe various channels, whereas the multiplexer 7 simply performs asummation of the signal components S₁, S₂, . . . , S_(M).

In the radio stage, a separator 8 reproduces the signal S on eachtransmission pathway corresponding to a radiating element 2 of theantenna 1. The phase-shifting unit 9 then applies a respective phaseshift D₁, D₂, . . . , D_(N) to the signal of each transmission pathway.Each phase shift is determined by the position in the antenna 1 of theradiating element 2, and depends on the direction of transmission of theantenna 1 controlled by the transmission pattern controller 10. FR-A-2792 116 describes an exemplary phase adaptation device usable as aphase-shifting unit 9.

The radio stage subsequently undertakes the conventional operations offiltering, of conversion to analogue 11, of transposition to the carrierfrequency 12 and of power amplification 13 on the basis of the signalsdelivered by the phase-shifting unit 9. Each radiating element 2 thenreceives from the amplifier 13 associated with it, by way of a duplexer14, the phase-shifted radio signal E₁, E₂, . . . , E_(N) correspondingto its transmission pathway.

Alternatively, the phase shifts could be applied to the signal of eachtransmission pathway in an analogue way, i.e. after conversion toanalogue 11.

The phase-shifting unit 9 can also perform a weighting of the amplitudeof the signal corresponding to each transmission pathway. In a mannerknown to the specialist in radio transmissions, such a weighting,jointly with phase shift law applied, makes it possible to modify awidth of the transmission pattern by altering the amplitudes of thesignals transmitted by each radiating element 2. Thus, during thetransmission of the signal by the antenna 1, the angular aperture of thetransmission pattern can be modified simultaneously with the fluctuationof the direction of transmission D.

An important advantage of such smart antennas is to make it possible tomaximize, for a given mobile terminal, the signal-to-interference ratio,by creating an antenna pattern whose “zeros”, that is to say locationswith very weak transmission or reception power, are in the direction ofthe interferers of the mobile terminal in question.

This principle is illustrated in FIG. 3 where two terminals 21 and 22are in communication with the base station 20. The lobes 23 and 24 ofthe antenna of the base station 20 are oriented mainly towards the twoterminals 21 and 22 respectively. The figure clearly shows that thesignal-to-noise ratio is maximized for each of the two terminals sincethe overlap of the lobes 23 and 24 is limited.

Theoretically and ideally, i.e. when the mobile terminals are uniformlydistributed around the base station and the angular spreading of themultiple paths is negligible, the use of a smart antenna as describedabove can reduce the interference level by the number of main lobes(e.g. by 2 in the example of FIG. 3).

Despite the interference reduction they imply, there is a problem withsmart antennas in that they also reduce the number of multiple pathseach mobile terminal can receive. Indeed, since they transmit mainly inone lobe with respect to a particular mobile terminal, they do notgenerate significant multiple paths in directions outside of the angularwidth of said lobe. The angular diversity is thus reduced.

An object of the present invention is to overcome this angular diversityreduction.

Another object of the invention is to alleviate fading, possibly whileensuring interference reduction simultaneously.

SUMMARY OF THE INVENTION

The invention proposes a method of transmitting from a base station to areceiver, the base station comprising first radiating means arranged fortransmitting according to an antenna pattern with respect to thereceiver and further comprising second radiating means arranged fortransmitting according to the same antenna pattern as the firstradiating means with respect to the receiver, the transmissions of thefirst and second radiating means being time shifted by a determinedduration.

Therefore, the receiver receives time shifted copies of the same signal,which allows it to determine the information transmitted moreefficiently, by virtue of time diversity, thus alleviating the fadingphenomenon.

The determined duration is advantageously set so that the correlationfactor between the transmissions of the first and second radiating meansis below a predetermined level. Alternatively or in addition, thedetermined duration can be set so that the inter-symbol interferencebetween the transmissions of the first and second radiating means isbelow a predetermined level. The determined duration can thus be chosenso as to achieve an acceptable compromise between these two constraints.

In a particular embodiment, the first and/or second radiating meanscould comprise a plurality of radiating elements, like in the case ofsmart antennas. In this situation, the radiating elements can bearranged for transmitting a signal with respective phase shifts, thephase shifts being set so as to define a direction of said antennapattern with respect to the receiver. Alternatively or in addition, theradiating elements can be arranged for transmitting a signal withrespective amplitude weights, the amplitude weights being set so as todefine a width of said antenna pattern with respect to the receiver.

Although the first and second radiating means could be the same (theradiating means thus transmits twice with a certain delay between bothtransmissions), most often they will be different.

Advantageously, the first and second radiating means can be spatiallyseparated, so as to provide space diversity and/or they can bedistinctly polarized, so as to provide polarization diversity. Thisensures a certain level of decorrelation between both transmissionswhich improves the quality of reception.

The invention also proposes a base station comprising first radiatingmeans arranged for transmitting according to an antenna pattern withrespect to a receiver and further comprising second radiating meansarranged for transmitting according to the same antenna pattern as thefirst radiating means with respect to the receiver, the transmissions ofthe first and second radiating means being time shifted by a determinedduration.

The preferred features of the above aspects which are indicated by thedependent claims may be combined as appropriate, and may be combinedwith any of the above aspects of the invention, as would be apparent toa person skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, already commented, represents a system of spherical coordinatesmaking it possible to characterize a direction of transmission orreception of the antenna of a base station;

FIG. 2, already commented, illustrates a control of the direction oftransmission of the antenna;

FIG. 3, already commented, is a diagrammatic representation of a smartantenna pattern;

FIG. 4 is a representation of a transmission system according to theinvention;

FIG. 5 is another representation of a transmission system according tothe invention, using adaptive array antennas; and

FIG. 6 illustrates a panel antenna which can be used according to theinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention is illustrated here in its application to a base stationof a radiocommunication system, in relation with a mobile terminal. Ofcourse, other kinds of transmitters or receivers could implement theinvention as well. In particular, the bases station could be the one ofa satellite system. As for the receiver, it could be another basestation, a base station controller, a radio network controller, etc.

FIG. 4 diagrammatically shows a base station according to the invention.The detailed processing means have not been reproduced in the figure,but they could be similar to the ones described with reference to FIG. 2for instance.

As shown in FIG. 4, the base station 29 comprises two radiating means 33and 34. These radiating means can have any form. For instance, theycould each consist in several antennas with respective orientations, ina single antenna (or a limited number of antennas) capable oftransmitting in several directions by virtue of mechanical means, suchas an articulated support, or in a plurality of radiating elements of anadaptive array antenna or smart antenna. In any case, each radiatingmeans should be arranged so as to be able to transmit according to adetermined antenna pattern with respect to a receiver.

Like in the prior art, the antenna pattern is preferably defined so asto maximize the signal-to-interference ratio for a given mobileterminal. In the illustrated example, the first group 31 of antennas 33is thus set so as to transmit in a main lobe 35 in the range of whichthe mobile terminal 36 is located.

According to the invention, the second group 32 of antennas 34 transmitsaccording to the same antenna pattern as the first group of antennas 33,with respect to the terminal 36. In other words, the second group ofantennas 34 transmits mainly in the lobe 35 in the range of which themobile terminal 36 is located.

The same signal y(t) is thus transmitted from the base station 29 to themobile terminal 36 via both the first group of antennas 33 and thesecond group of antennas 34. But, the signal is delayed (reference 30)by a determined duration τ before being transmitted by the second groupof antennas 34. By contrast, the signal y(t) is transmitted by the firstgroup of antennas 33 without any delay. The transmissions of the firstand second groups of antennas are thus time shifted by the duration τ.

Of course, if the mobile terminal 36 moves, the antenna pattern can bemodified accordingly to track the mobile terminal. In practice, the maindirection of the lobe 35, and possibly its width, will thus change.

Such operation generates time diversity, since the mobile terminal 36gets several copies of the signal y(t). This compensates to a certainextent the possible limitation of multiple paths introduced by theantenna, like in the case of a smart antenna for instance. But it canpreserve other properties of the antenna, like the interferencereduction in the case of smart antennas.

Another advantage of the invention is that it does not necessarily implychanges in the mobile terminal. Indeed, mobile terminals generally havemeans for receiving and possibly combining different copies of atransmitted signal (e.g. for determining a signal received over multiplepaths) and are thus able to receive a signal transmitted according tothe present invention.

Advantageously, the duration τ can be set so that the transmissions ofthe first and second groups of antennas are sufficiently decorrelated,i.e. the correlation factor is below a certain level. This can beachieved by analysing the impulse response of the channel over which thesignal y(t) is transmitted. More particularly, a distribution of themultiple paths can be built over time, so as to determine the durationafter which the most significant copies of the signal have been receivedby the mobile terminal. For example, the time t could be determined asthe time below which 90% of the most significant copies of the signalhave been received. The duration τ could thus be chosen to be more thant or equal to t. Of course, the duration τ may also differ depending onthe modulation used between the base station 29 and the mobile terminal36.

It can be shown that a duration τ of two bits, i.e. 7.38 μs, ensures agood level of the decorrelation between both transmissions in a GSMradio system. In a UMTS radio system, a duration τ of only one chip,i.e. 0.26 μs, would be suitable as well.

On the other hand, the duration τ can be set so that the inter-symbolinterference between both transmissions is below a predetermined level.Indeed, the signal y(t) is sent over a radio channel in the form ofsymbols. If both copies of y(t) sent by each group of antennas are tootime shifted, symbols sent by the second group of antennas 34 could bereceived by the mobile terminal 36 in a time range in which latersymbols are received from the first group of antennas 33. In thissituation, the mobile terminal could have difficulties in retrieving thesymbols sent. The exemplary values mentioned above can ensure a correctlevel inter-symbol interference.

Of course, one skilled in the art could determine another duration τ inorder to limit the inter-symbol interference and/or to ensure a certainlevel of decorrelation between the successive transmissions. Thisdetermination could be based on a theoretical approach or on astatistical approach by monitoring some counters or performance criteriaas well known in the art.

FIG. 5 is another representation of a transmission system according tothe invention, in an example in which the first and second groups ofantennas are adaptive array antennas, i.e. smart antennas. Theillustrated base station comprises a first group 38 of radiatingelements 39 and a second group 37 of radiating elements 40. Eachradiating element, which may be of any type (large antenna, elementarysource, dielectric focusing source, cross-polar antenna, etc), has aspecific contribution within a transmission. The contributions generallydepend on the angular position of the mobile terminal involved in thetransmission with respect to the antenna. Thus, it is possible to send asignal at antenna level in a manner which favours the direction of themobile terminal.

The signal y(t) to be transmitted to the terminal 41 by the first group38 is sent partially by each of the N radiating elements 39 (N>1). Thesignal y(t) is replicated into N signals weighted by respectivecoefficients w_(i) (1≦i≦N) before being applied to the radiatingelements.

In parallel, the signal y(t−τ), i.e. the signal y(t) time shifted by theabove mentioned duration τ, is transmitted is sent partially by each ofthe N radiating elements 40 of the second group 37. The signal y(t−τ) isreplicated into N signals weighted by respective coefficients w′_(i)(1≦i≦N) before being applied to the radiating elements 40.

The coefficients w_(i) are set so that the first group 38 of radiatingelements 39 transmits according to an antenna pattern directed to theterminal 41. The coefficients w′_(i) are set so that the second group 37of radiating elements 40 transmits according to the same antenna patternas the first group 38, directed to the terminal 41. In the illustratedexample, the coefficients w_(i) and w′_(i) are equal to each other. Ofcourse, they could also differ from each other if requested to obtain acommon antenna pattern for both groups of radiating elements. This canbe the case, e.g. when the first and second groups of radiating elementsare distant from each other or oriented differently.

In a particular example, the coefficients w_(i) and w′_(i) are complexcoefficients whose argument depends on the angular position θ of themobile terminal 41 with respect to the antenna. If d_(i) denotes thedistance, calculated as a number of λ/2 (λ being the radio wavelength),which separates the (i+1)^(th) radiating element from the firstradiating element of the first group 38 of radiating elements 39, thesetwo elements send with a geometrical phase shift of 2π·d_(i)·cos(θ)/λ.The base station may thus take the coefficients w_(i) of the form:

$W_{i} = {\frac{1}{\sqrt{N}} \times {^{{- {`j\pi}} \cdot d_{j} \cdot {\cos {(\theta)}}}.}}$

A similar expression may be used with regard to the coefficients w′_(i).

However, other expressions of the contributions w_(i) and/or w′_(i) maybe used, in the case of adaptive array antennas, e.g. to minimize thecontributions of the other signals in directions other than the usefulsignal.

As mentioned above, not only the respective phase shifts, but also therespective amplitude weights of the signal sent could be different foreach radiating element 39 or 40, so as to define a determined width ofthe antenna pattern with respect to the mobile terminal 41.

At the mobile terminal level, the two copies of y(t) (i.e. y(t) andy(t−τ)) are taken into account in order to determine the informationsent by the base station. This can be achieved by combining the twocopies as it is usually done for multiple paths. If the mobile terminalis provided with a rake receiver whose fingers detect different copiesof a signal, one or several fingers of the rake could be used for thereception of the signal y(t) sent by the first group 38 of radiatingelements 39, whereas another or several others fingers of the rake couldbe used for the reception of the signal y(t−τ) sent by the second group37 of radiating elements 40.

It can be noted that the antennas as described hitherto make it possibleto send, but also to receive in a way which is more favourable in thedirection of the relevant mobile terminal. When receiving, that is tosay in the uplink sense, from the mobile terminal 41 to the basestation, the radiating elements are used as sensors. Signals are pickedup by these elements and are subsequently weighted by respectivecoefficients and also w′_(i) if both groups of antennas are used forreception. The signals thus weighted can then be summed to retrieve theuseful signal y(t) (possibly after a combination of the retrievedsignals y(t) and y(t−τ)).

Advantageously, the first and second radiating means provide a furthertype of diversity, so that their respective transmissions aredecorrelated. For instance, space diversity can be carried out byspatially separating the first and second radiating means as will beappreciated by one skilled in the art. Alternatively or in addition,polarization diversity could be used by distinctly polarizing the firstand second radiating means. This case is illustrated in the exampledescribed below with reference to FIG. 6.

In this example, the base station comprises a panel antenna 42 providedwith a plurality of cross dipoles 43. Each cross dipole includes aradiating element with a +45° polarization and a radiating element witha −45° polarization, i.e. orthogonal polarizations. As more particularlyshown with regard to the dipole 44, each radiating element of the dipolereceives a different part of the signal to be transmitted via arespective cell 45 or 46. The cell comprises an amplifier 47 or 49 whichcan be followed by a duplexer 48 or 50. The amplification is thusperformed very close to the corresponding radiating element in thisexample.

Of course, the total power of the signal to be transmitted is sharedbetween both radiating elements constituting the dipole. The power canbe equally shared between both radiating elements constituting thedipole. It is also possible to distribute the total power of the signalto be transmitted differently between both radiating elementsconstituting the dipole. For example, the transmission power can be setaccording to the reception power on each radiating element constitutingthe dipole. The power distribution can thus be estimated on the uplinkand then applied to the downlink. This technique can be used not only inthe case of FIG. 6. But it is particularly useful when the radiatingelements are polarized differently, because the different transmissionpowers can thus compensate the different polarizations in a way.

With this architecture, it could be decided that all the radiatingelements with a +45° polarization are provided with a non-delayed signalto be transmitted to a receiver, whereas all the radiating elements witha −45° polarization are provided with a delayed signal to be transmittedto a receiver, according to the principles described above. Therefore,the two time shifted transmissions use orthogonal polarizations, thusproviding a better decorrelation with each other.

1-24. (canceled)
 25. A method of transmitting from a base station to areceiver, the base station comprising a first antenna assembly and asecond antenna assembly, the method comprising: transmitting, by thefirst antenna assembly having plural radiating elements, a signalaccording to an antenna pattern with respect to the receiver;transmitting, by the second antenna assembly having plural radiatingelements, the signal with a time delay according to the same antennapattern as the first antenna assembly with respect to the receiver,wherein the signal and the signal with the time delay transmitted by therespective first and second antenna assemblies are time shifted withrespect to each other by a determined duration, wherein the signals arechannel coded, interleaved, modulated and allocated to time slotsintended for one or more receivers in a time division multiple access(TDMA) scheme.
 26. The method of claim 25, wherein the first and secondantenna are arranged to achieve optimal decorrelation thoughpolarization diversity.
 27. The method of claim 26, wherein the firstand second antenna assemblies are polarized by having a firstpolarization is at a value substantially equal to +45 degrees and asecond polarization at a value substantially equal to −45 degrees. 28.The method of claim 25, wherein said determined duration is set so thata correlation factor between transmissions of the first and secondantenna assemblies is below a predetermined level.
 29. The method ofclaim 25, wherein said determined duration is set so that theinter-symbol interference between transmissions of the first and secondantenna assemblies is below a predetermined level.
 30. The method ofclaim 25, wherein the radiating elements of each of the first and secondantenna assemblies are arranged for transmitting signal components withrespective phase shifts, the phase shifts being set so as to define adirection of said antenna pattern with respect to the receiver.
 31. Themethod of claim 25, wherein the radiating elements of each of the firstand second antenna assemblies are arranged for transmitting signalcomponents with respective amplitude weights, the amplitude weightsbeing set so as to define a width of said antenna pattern with respectto the receiver.
 32. The method of claim 25, wherein the first andsecond antenna assemblies are spatially separated, so as to providespace diversity.
 33. The method of claim 25, wherein each radiatingelement of the first antenna assembly is coupled to a respectiveradiating element of the second antenna assembly, so as to form a crossdipole with orthogonal polarizations.
 34. The method of claim 25,wherein the base station is comprised in a digital radio communicationsystem.
 35. A base station comprising: a first antenna assembly havingplural radiating elements arranged for transmitting a signal accordingto an antenna pattern with respect to a receiver; a second antennaassembly having plural radiating elements arranged for transmitting thesignal with a time delay according to the same antenna pattern as thefirst antenna assembly with respect to the receiver, wherein the signaland the signal with the time delay transmitted by the respective firstand second antenna assemblies are time shifted with respect to eachother by a determined duration, wherein the signals are channel coded,interleaved, modulated and allocated to time slots intended for one ormore receivers in a time division multiple access (TDMA) scheme.
 36. Thebase station of claim 35, wherein the first and second antenna arearranged to achieve optimal decorrelation though polarization diversity.37. The base station of claim 36, wherein the first and second antennaassemblies are polarized by having a first polarization is at a valuesubstantially equal to +45 degrees and a second polarization at a valuesubstantially equal to −45 degrees.
 38. The base station of claim 35,wherein the determined duration is set so that a correlation factorbetween transmissions of the first and second antenna assemblies isbelow a predetermined level.
 39. The base station of claim 35, whereinthe determined duration is set so that the inter-symbol interferencebetween transmissions of the first and second antenna assemblies isbelow a predetermined level.
 40. The base station of claim 35, whereinthe radiating elements of each of the first and second antennaassemblies are arranged for transmitting signal components withrespective phase shifts, the phase shifts being set so as to define adirection of said antenna pattern with respect to the receiver.
 41. Thebase station of claim 35, wherein the radiating elements of each of thefirst and second antenna assemblies are arranged for transmitting signalcomponents with respective amplitude weights, the amplitude weightsbeing set so as to define a width of said antenna pattern with respectto the receiver.
 42. The base station of claim 35, wherein the first andsecond antenna assemblies are spatially separated, so as to providespace diversity.
 43. The base station of claim 35, wherein eachradiating element of the first antenna assembly is coupled to arespective radiating element of the second antenna assembly, so as toform a cross dipole with orthogonal polarizations.
 44. The base stationof claim 35, wherein the base station is comprised in a digital radiocommunication. system.